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      <h1 class="f1 athelas mt3 mb1">记录一次C&#43;&#43;实验</h1>
      
      
      
      <time class="f6 mv4 dib tracked" datetime="2021-01-12T22:41:33Z">January 12, 2021</time>
      

      
      
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    <div class="nested-copy-line-height lh-copy serif f4 nested-links nested-img mid-gray pr4-l w-two-thirds-l"><p>今天有一个猜想：c++函数局部构造的class/struct，如果要返回给父函数，是通过移动语义实现的。
但我并不十分确定，于是做了个实验验证之。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-c++" data-lang="c++"><span style="color:#75715e">#include</span> <span style="color:#75715e">&lt;iostream&gt;</span><span style="color:#75715e">
</span><span style="color:#75715e"></span><span style="color:#66d9ef">using</span> <span style="color:#66d9ef">namespace</span> std;

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">A</span> {
<span style="color:#66d9ef">public</span><span style="color:#f92672">:</span>
  A() <span style="color:#f92672">:</span> a{<span style="color:#ae81ff">0</span>} { cout <span style="color:#f92672">&lt;&lt;</span> <span style="color:#e6db74">&#34;constructing A</span><span style="color:#ae81ff">\n</span><span style="color:#e6db74">&#34;</span>; }
  A(<span style="color:#66d9ef">const</span> A<span style="color:#f92672">&amp;</span> other) <span style="color:#f92672">:</span> a{other.a} { cout <span style="color:#f92672">&lt;&lt;</span> <span style="color:#e6db74">&#34;copy constructing A</span><span style="color:#ae81ff">\n</span><span style="color:#e6db74">&#34;</span>; }
  A(A<span style="color:#f92672">&amp;&amp;</span> other) <span style="color:#f92672">:</span> a{other.a} { cout <span style="color:#f92672">&lt;&lt;</span> <span style="color:#e6db74">&#34;move constructing A</span><span style="color:#ae81ff">\n</span><span style="color:#e6db74">&#34;</span>; }

  <span style="color:#66d9ef">void</span> <span style="color:#a6e22e">goo</span>() { a <span style="color:#f92672">=</span> a <span style="color:#f92672">*</span> <span style="color:#ae81ff">2</span>; cout <span style="color:#f92672">&lt;&lt;</span> <span style="color:#e6db74">&#34;calling goo</span><span style="color:#ae81ff">\n</span><span style="color:#e6db74">&#34;</span>; }
<span style="color:#66d9ef">private</span><span style="color:#f92672">:</span>
  <span style="color:#66d9ef">int</span> a;
};

A <span style="color:#a6e22e">foo</span>(A x) {
  x.goo();
  <span style="color:#66d9ef">return</span> x;
}

<span style="color:#66d9ef">int</span> <span style="color:#a6e22e">main</span>() {
  A x;
  A y <span style="color:#f92672">=</span> foo(x);
  <span style="color:#66d9ef">return</span> <span style="color:#ae81ff">0</span>;
}
</code></pre></div><p>不加任何优化选项，编译运行。
输出结果如下：</p>
<blockquote>
<p>constructing A <!-- raw HTML omitted -->
copy constructing A <!-- raw HTML omitted -->
calling goo <!-- raw HTML omitted -->
move constructing A</p>
</blockquote>
<p>结果验证了我的猜想，即：返回局部构造的class/struct时，是移动语义，调用的是移动构造函数（前提是存在移动构造函数，如果只实现了拷贝构造函数，那么移动构造函数就不存在，这时只能调用拷贝构造函数）。
目的是将子函数构造的对象转交给父函数，从高层语义上讲，调用移动或拷贝构造函数应该都是可以的。但是，由于局部变量在函数结束时需要销毁，如果先拷贝再把自身销毁，效率有亏。而直接将自己掌握的资源移动给父函数，效率更高。所以，至少在g++编译器实现上，采用的是移动语义。至于c++标准对此有无规定，尚不得而知。</p><ul class="pa0">
  
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